1. Field of the Invention
The present invention relates to an improved method and apparatus for burning hazardous waste in a rotary device such as a rotary cement kiln and to the improved method and apparatus for feeding materials to a rotating cylinder process.
The apparatus and feeding system disclosed herein are applicable to any situation where a need exists to feed material into a rotating cylinder at a point intermediate its ends in a continuous and controlled manner. Rotary industrial furnaces such as cement kilns, aggregate kilns, lime kilns, pulp and paper recovery kilns and incinerators may utilize the feeding system to transfer fuel material into the furnace at strategic locations which may enhance the manufacturing process or permit the utilization of fuel grade materials which heretofore was not possible. The application of the feeding system is not, however, restricted to the feed of fuel grade material alone. The system is equally capable of metering raw material or chemical additives into rotating cylinders at desired strategic locations. Kiln dust, fly ash and many other raw materials can be beneficially utilized by the continuous introduction of such materials directly into the calcining zone of a cement kiln. Horizontal rotary reactors and misers utilized in the chemical industry can use the feeding system to introduce chemical additives at strategic steps in the reaction process.
The range of application for the improved rotary kiln feeding apparatus disclosed herein is quite broad. The preferred embodiment disclosed, however, is specifically designed to be utilized by cement kilns and aggregate kilns in the feeding and burning of waste material as a means of thermal destruction and energy recovery.
Cement kilns are typically large rotary furnaces that range in length between 150 and 550 feet. Typical kiln diameters range between 10 and 15 feet. They are slightly inclined from horizontal so that minerals that are fed in the upper end will slowly travel the length of the kiln and be discharged at the lower end. Kilns typically rotate between 30 and 120 revolutions per hour with 60 being a good average. The lower end of the kiln is typically equipped with a primary burner in which fuel is burned to produce a heat source having temperatures in the range of 3000 to 3500.degree. F. A typical mineral feed stock used to make cement is composed of limestone (calcium carbonate), sand or shale (silica source), clay (alumina source) and iron ore (iron source). As much as 90% of the raw material fed can be limestone. As the minerals move through the kiln, they are slowly heated and they pass through three distinct zones. The first, or preheat zone, has mineral temperatures that range from 350 to 1500.degree. F. The second, or calcining zone, has mineral temperatures that range from 1500 to over 2100.degree. F. This is the point where the minerals become hot enough for the carbon dioxide to dissociate from the limestone. The resulting calcium oxide is very reactive and it begins to chemically bind with the other minerals at this point. The chemical reactions that take place in this zone are endothermic (i.e., they require heat to occur). The last and hottest zone is the sintering zone. This is the zone closest to the heat source, and the minerals must reach temperatures of approximately 2600 to 2700.degree. F. in order for the final chemical reactions to occur to produce the calcium silicate material known as "clinker." The chemical reactions in this zone are exothermic (i.e., they liberate heat). Clinker is a substance similar to lava which is produced in marble-size pellets which exit the lower end of the kiln. The clinker is then cooled and ground with small amounts of gypsum to produce the product known as Portland cement.
There is a wet process and a dry process for producing cement. In the wet process, minerals are ground and slurried with water before entering the kiln. Wet kilns must evaporate the water from the minerals before preheating can occur. For this reason they are typically 450 to 550 feet in length. The calcining section is located approximately one-third of the length from the fired end, and it is typically 150 feet or more in length. Wet kilns are considered older technology and they are more energy intensive than the newer dry kiln technology. A typical wet kiln burns 5.5 million BTUs to produce a ton of clinker, while a typical dry kiln burns 4 million BTUs per ton of clinker. Wet kilns have been motivated to burn alternative fuels as a way of reducing their energy cost so that they can continue to compete with dry kilns.
Dry kilns occur in many forms. The earlier dry kilns operated in the same way as wet kilns except the minerals were introduced in dry powder form. Long dry kilns of this type are more energy efficient than wet kilns for obvious reasons. Newer dry kilns employ a vertical preheating tower prior to entry of the minerals into the kiln itself. Hot combustion gases flow from the kiln upward through the preheat tower while dry powdered minerals cascade downward. The resulting countercurrent flow of combustion gases preheats the minerals prior to entry into the kiln and, for this reason, preheater kilns have a much shorter rotary kiln in the range of 150 to 250 feet in length. In preheater kilns, the calcining zone begins at the point of entry into the rotary kiln itself. A further improvement has been made to the preheater technology by introducing hot combustion gases from a separate burner mounted in the first stage of the preheat tower. With this arrangement, calcining of the minerals begins in the preheater tower and it finishes in the kiln. Precalcining kilns represent the newest technology and they are the most energy efficient. They are the only kilns in which calcining zone temperatures in excess of the required 1800.degree. F. are accessible from a stationary point at the riser duct connecting the kiln to the preheater tower. All other kilns achieve these temperatures within the rotating kiln itself.
Combustion gases leaving the kiln typically contain from 6 to 30% of the feed minerals as dust. Particulate emissions are typically controlled with electrostatic precipitators or fabric filters and they are often recycled back into the kiln. Some kilns use a bypass duct to divert from 5 to 30% of the kiln off-gases to a separate air pollution control system which collects dust but does not return it to the process. This is done to prevent the build-up of metal salts inside the kiln which can affect product quality.
As set forth above, cement kilns function as excellent incinerators for a number of reasons. First, temperatures inside the kiln exceed the 1800.degree. F. determined by the Environmental Protection Agency to be necessary for complete thermal destruction. Residence time of combustion gases inside the kiln far exceed the required 2 seconds as well. The turbulent flow of alkaline mineral dust within the combustion gases flowing down the kiln provides excellent scrubbing of the gases before they are discharged to the environment. Finally, the huge mass of reactive minerals traveling down the length of the kiln chemically binds with inorganics fed and it also provides a stabilizing effect to the process as well. This means that incineration of material in cement kilns produces no residue in comparison to RCRA units which produce toxic ash that must be landfilled. Also, cement kilns burn fuel to produce a valuable commercial product while incinerators consume fuel only to destroy hazardous waste. Burning hazardous waste in cement kilns produces no net increase in emissions and in most cases emissions are reduced because of the lower content of sulfur and other chemicals in the waste. In comparison, incineration represents a net increase in emissions to the atmosphere.
The role of the cement industry in the thermal destruction of liquified hazardous waste has long been recognized. Most hazardous organic liquids are now burned as fuel in cement kilns as a means of reducing energy cost and as an income producer. In 1990, the cement industry consumed approximately 300 million gallons of liquid hazardous waste. The generation of fuel grade liquid hazardous waste is expected to increase substantially within the next 3 to 5 years but the industry already has adequate capacity to handle the increases. The success of thermal destruction of liquid hazardous waste in cement kilns is well proven and the industry has effectively captured this market from the RCRA incinerators. The next market opportunity for cement kilns is burning organic sludges and solids that heretofore has been the domain of the incinerators and landfills.
There are several problems, however, associated with the prior art that has deterred its widespread use. First, the prior art technology requires that waste be shredded and packed into 6 gallon pails or similar containers. This requirement causes waste processors to spend large sums of money for materials and labor to containerize material. The process is also slow and laborious and it has proven impractical to containerize large quantities of material. Second, the technology involves the injection of one or two pails into the kiln per revolution. This intermittent introduction of fuel packs results in irregular releases of energy and emissions thereby threatening the stable conditions within the kiln. The consistency of fuel quality also varies from pail to pail so that consistent combustion conditions are difficult to maintain. Finally, the technology requires the kiln to intake excess air at the front end of the kiln to ensure that there is adequate oxygen in the calcining zone to promote complete combustion of the solid fuel. This situation reduces the overall thermal efficiency of the kiln.
The feeding system of the present invention has overcome the disadvantages of the prevailing technology in a number of ways. First, the system allows for the direct feed of material into the calcining zone without the need for containerization. The system is designed to feed truck load quantities of material that has been pre-shred to a 3 to 4 inch diameter particle size. The system is equally capable of feeding shred drums (including drum metal), API separator sludge, contaminated soil, tire chips, and virtually anything that can be shred. This makes it possible to handle large quantities of bulk material that has proven impractical with the prevailing technology. The system continuously mixes the bulk material so that the properties of the fuel remain consistent throughout the feed cycle. Also the system allows the simultaneous feed of material from two sources. By feeding tire derived fuel (or any consistent Btu material) simultaneously with hazardous waste, the Btu fluctuations of the hazardous waste can be leveled to provide a constant Btu input into the kiln regardless of the type of material being utilized. This promotes consistent conditions within the kiln which are desired by the kiln operators. Also, simultaneous feeding enables Btu input into the kiln to be maintained constant even if the hazardous waste feed system in instantaneously shut down. This feature is yet another way to guarantee consistency.
Second, the system disclosed herein feeds the material into the kiln in a continuous fashion that is completely controlled by the kiln operator. This continuous, and controlled, feeding promotes steady state conditions within the kiln and consistent replacement rates in the 40% range will be easily maintained. It is also anticipated that the continuous feed will allow even heavier loading of inorganics in the waste derived fuel. The system further reduces the particle size of material fed so that the final consistency entering the kiln is extremely fine and combustion air is provided directly in proportion to the feed rate of material. These factors create optimum conditions necessary to ensure complete combustion of the fuel and the overall thermal efficiency of the kiln is preserved since excess combustion air need not be fed at the primary burner. The system is protective of human health and safety through the use of inert gas blanketing and extensive monitoring throughout. All of these factors enable the technology to handle a broad range of solids and sludge materials in a practical and environmentally sound manner.
The feed system can be furnished with additional equipment to enable the pre-processing of waste obtained directly from generators. The pre-processing system shreds and blends both drummed and bulk waste to produce a consistent fuel suitable for feeding into the kiln. Even liquids and sludges can be blended with solids to produce a type of "mud" suitable for feeding. With this system, waste can be routed directly to the kiln without the need for other facilities to process and blend the waste. Without the pre-processing system, the feed unit is dependent upon other facilities to pre-process the material prior to arrival on site.
2. Brief Description of Related Art
Cement kilns used in the production of cement are quite large with typical cylinders being between 250 and 750 feet long and up to 25 feet in diameter. They are tilted slightly and rotate slowly. They have been approved and have been used for the burning liquid hazardous waste. A good summary of the state of the industry with respect to the use of hazardous waste for fueling cement kilns is the article entitled "Cement Kilns 1990" published in Environmental Information Digest, June 1990, pages 14 to 23.
U.S. Pat. No. 2,002,972 discloses an apparatus for making portland cement and is directed to conducting the cement mixture through piping extending around the rotating cylinder and positioned to scoop up the mix and allow it to pass through the piping and be discharged into the interior of the rotating cylinder and this structure prevents back firing from the kiln into the hopper. The kiln is fired by a gas, oil or coal burner which discharges hot gases into the discharge end of the rotating cylinder.
Other examples of rotary kilns and rotary feeders and crushers are disclosed in U.S. Pat. Nos. 1,813,061, 2,195,815, 3,180,501 and 3,542,217.
U.S. Pat. No. 4,435,081 discloses a concrete mixing plant with elevator wheels which includes a rotating drum having feeding means for cement and aggregates and an elevator wheel suitable for premixing aggregates.
U.S. Pat. No. 4,870,911 discloses a method and apparatus for waste disposal including a rotating kiln into which the waste material is discharged by a screw feeder and heat is provided by the plasma torch mounted in the gas outlet duct. The mounting shaft for the rotating kiln is hollow so that materials may be introduced or taken from the interior of the kiln.
U.S. Pat. Nos. 4,850,290, 4,930,965 and 4,969,407 disclose method and apparatus for the disposal of solid hazardous waste by packaging the solid waste in sealed containers which are delivered into the rotary kiln as fuel modules.